This invention relates to measuring the shape of surfaces. Whereas other uses can be envisioned, the invention has particular applicability to measurement of the corneal surface and to facilitating treatments of the eye.
Corneal topography measurements are valuable for planning, performing, and evaluating the effects of surgical procedures. Measurements of the corneal surface are needed for keratorefractive procedures, which correct a refractive power of the eye by changing the curvature of the corneal surface. In addition, corneal topography can also be used to predict the results of radial keratotomy, evaluate the design of epikeratophakia for myopia, diagnose the stage of keratoconus, and guide suture removal following corneal transplantation.
There are several methods to test and characterize the optical power of the eye and the cornea in particular. One of the oldest methods is the Snellis diagram test, therein a patient is asked to read letters or to recognize shapes from a standard distance. This is a subjective method which requires the patient's cooperation.
Since the corneal curvature and its dioptric power account for about three quarters of the refractive power of the eye, it is important, however, to measure the corneal surface with greater accuracy than the Snellis diagram test provides.
One class of methods of measuring the corneal surface is based on the deflectometry principle, which utilizes reflection of light from the smooth corneal tear film (i.e., the lower, oily part of the tear film). In this method, a system of rings is optically projected onto the surface of the eye. A doctor directly observes the symmetry of the reflected rings and judges the condition of the eye. This qualitative technique is quite reliable; however, it is dependent upon the doctor's experience.
In recent years, automatic measurement devices which measure the shiny surface of the cornea using deflectometry have been introduced. These are computerized systems which analyze distortion of the images of a system of rings optically projected towards the eye and detected by a camera detection system. The spatially defined system of rings is projected onto the smooth eye surface from a precisely positioned source governed by a computer. The reflected pattern is detected by a camera and stored in the memory of a computer. Using the well-defined characteristics of the incident and detected light, the geometric position of the source and the detector, and the shapes of the incident and detected pattern the computer calculates the shape of the reflecting sphere.
Such a computerized topographer can be used as a principal guide in a laser system performing corneal sculpturing surgery, to provide the necessary pre-operative and post-operative corneal measurements, or to provide the measurements to guide post-operative manipulation of the cornea for reduction of astigmatism. However, during and after eye surgery, once the epithelium is removed from the eye surface, the local microtopology of the eye surface has changed so that the surface of the eye is only partially a specular reflector, and now partially a diffuse reflector. Since a diffuse reflector has no fixed relationship between the incident angle and the reflected angle of the projected light, the described deflectometry-based topographer is no longer useful. Furthermore, since both the corneal topographer using deflectometry and a laser beam delivery system of a laser sculpturing system require positioning on the optical axis of the eye, there is difficulty in incorporating both systems into one unit designed for intraoperative use. In addition, since the vision of a patient during surgery or after de-epithelization is significantly impaired, it is difficult to achieve proper eye alignment necessary for deflectometric measurement.
Rasterography or fringe phase shifts are methods of determining topography of the cornea which are well suited for diffusive surfaces. The method does not require smooth reflective surfaces and images can be obtained on surfaces with some degree of epithelial irregularity. The methods use an optical pattern, for example, a grid of vertical and horizontal bars of light projected onto the corneal surface. The projected pattern has very well established characteristics including shape, regularity, and separation of the points. A detection system registers and analyzes the deformation of the detected pattern. A computer analyses the deformation data and establishes the topography of the measured surface. The detection system can be located in any place since it detects the light from a diffused reflector which reflects light in all directions. The advantage of this method is that the projected image can cover the entire cornea including the central visual access, far periphery, and limbus, interpalpebral conjunctiva, and lid margins. This technique, however, is not useful for smooth, shiny surfaces, such as the epithelium surface.
There are other optical methods such as confocal microscopy, shared interferometry, infrared interferometry and multi-color interferometry that can be used to characterize the eye surface but each has its limitations and fails to meet fully, for instance, the needs in the case of laser sculpting of the cornea.
Again, as suggested above, in laser sculpting of the cornea, the devices based on deflectometry are well tailored to measure the specular type surface which is the surface of the eye during the initial stages of laser sculpting procedures, and devices based on rasterography are well suited to measure diffuse type corneal surface which occurs after laser sculpturing of the corneal surface was performed, but presently, there are no entirely satisfactory devices which can precisely measure both types of surfaces, and particular surface which in part are of one type and in part another. Neither are there devices which can be conveniently integrated into surgical laser systems. Furthermore, some of the previously mentioned instruments require a patient's cooperation since he or she needs to look in some specific direction.
In general, the discussed topographers are based on the assumption that the cornea has a conic surface i.e. a sphere, an ellipse, a parabola, or a hyperbola, but in reality, the living cornea is none of these; it is an aspheric section with great individual variation, and hence most of the known techniques are not completely accurate.